Sensorless motors have been increasingly used in, for example, MD drives and HDD drives, because of their simplicity in structure having no rotational position sensor that utilizes, for example, a Hall element. However, sensorless motors have a problem in that they are not always started up in a predetermined direction due to the fact that the position of the magnetic rotor of a stationary motor is not known.
In a conventional method of starting up a sensorless motor, the motor is forcibly vibrated in a predetermined sequence that has nothing to do with the position of the rotor.
This startup method suffers a problem in that the motor is not always started in the normal (or forward) direction due to the fact that the motor is supplied with three-phase voltages generated on the basis of a startup logic that has nothing to do with the position of the rotor.
Japanese Patent Application Laid Open No. H5-268791 (hereinafter referred to as Patent Document 1) discloses a motor drive unit adapted to detect the position of a rotor by monitoring the zero crossover voltage in the stator coil thereof in a floating state. This method allows detection of the position of the rotor in the process of re-synchronizing the motor after it has lost synchronization. However, the method cannot detect the position of the rotor before the motor is started up, since the method can detect the position of the rotor only if the motor is in motion.
In view of these circumstances, the applicants of the present invention have proposed a motor drive unit capable of properly starting up a sensorless motor based on a predetermined startup logic in accordance with the rotational position of the rotor at rest. (See Japanese Patent Application Laid Open No. 2004-104846, which will be referred to as related reference.) This related reference is not in the public domain as of the priority date of this application,
The motor drive unit of the related reference is adapted to sequentially supply the stator coils of a sensorless motor with drive voltages (referred to as rotor-position detection drive voltages) that are insufficient to drive the motor at rest, and compares the middle point voltage of the stator coils with a given reference voltage to provide results of the comparison (referred to as detection pattern), from which the position of the rotor is detected. A proper startup logic is determined based on the position of the rotor.
In the motor drive unit of the related reference, middle point voltage is generated in a different condition from one motor to another (different in level and variation for example). Thus, in order to obtain predetermined detection patterns for different types of motors, the reference voltage must be adjusted for each motor used. Moreover, depending on the conditions of the stator coils generating the middle point voltage, an anticipated detection pattern cannot be obtained by adjusting only the reference voltage.
Further, when the power supply voltage applied to the motor has dropped below, or risen above, the rated voltage thereof, the middle point voltage of the stator coils gets changed according to the amount of drop or rise in the power supply voltage. Thus, under such condition, it is still more difficult to obtain a prescribed result by comparison (i.e., detection pattern).
It is therefore, an object of the present invention to provide a motor drive unit for use with a sensorless motor capable of unfailingly detecting the position of the stationary rotor and determine an appropriate startup logic therefor, irrespective of the type the sensorless motor, thereby capable of determining a proper startup logic and enabling the motor to start up in a stable manner. It is another object of the invention to provide an electronic apparatus equipped with a sensorless motor driven by the motor drive unit of the invention.
It is a further object of the present invention to provide a sensorless motor that can be started up in a stable manner even in the event of an excessive drop or rise in the power supply voltage supplied to the motor.